This invention generally relates to reactive sputtering systems in which a plasma effects some processing on a substrate. Specifically, the invention has application to DC sputtering when coating with some insulating materials formed by chemical reaction in the coating process. It also involves power supply designs used in such applications.
The field of DC plasma processing is one which is well known. In these processes, a DC power supply creates an electric potential between a cathode and anode and thereby creates a plasma. In the deposition mode, the plasma then acts upon a material target to create a thin film on some substrate. This thin film may either be comprised of the target material itself or may be the result of some reaction with some element within the coating chamber. It is the latter case, called "reactive sputtering" with which the present invention is most concerned. Naturally both the materials and elements involved and the specific applications vary greatly. Applications may range from coating architectural glass to the creation of micro chips.
One of the challenges in many such applications is that electrical discharges or arcs can occur. This is particularly true in reactive sputtering when the reactive product is an insulator, such as aluminum oxide (Al2O3). As one example, this type of coating process is particularly challenging because it involves forming insulating regions on the material target and bombarding them with ions. This leads to charging of the insulating regions which can cause electrical breakdown of the insulator. As a result, the electrical environment during reactive plasma processing can be particularly conducive to arc discharges. These arc discharges are undesirable not only because they represent potential non-uniformities in the coating process, but also because they can lead to the creation of particulates which can in turn create defects in sensitive items such as computer disks and integrated circuits. Also, it may be necessary to protect the power supply from arc discharges by turning it off momentarily, and this can adversely affect throughput. Similarly, the processing itself can be adversely affected by such insulating regions. This can have a number of results ranging from changing the characteristics of the thin film to affecting the rate or nature of the processing. In this regard the process itself is very empirical. Often these effects, and their solutions, are unpredictable and are achieved after some trial and error without a complete understanding of the exact impact on the process.
With respect to the observable problem of arc occurrences, this aspect has been well known to those skilled in the art, and has been addressed with limited success in ordinary metallic sputtering and reactive sputtering of films that are not too insulating. Initially it was common to completely shut down the process and perhaps even clean the chamber before restarting. In other instances, lower processing rates were used to make the occurrences of arcs less frequent. More recently, it has been attempted to divert the arc by quickly shutting off the supply of power to the plasma itself. Unfortunately, most such solutions acted only after damage had been done and thus served to minimize, but not completely avoid, problems in more sensitive processing environments. In order to react as quickly as possible, switch-mode or low energy storage power supplies have also been used for many applications. In spite of the fact that they inherently store less power and thus can be manipulated to minimize the negative effects of such arc occurrences, their use alone has not been sufficient for many processing environments. Interestingly, solutions by component designers have often been utilized without full explanation to those involved in the processing itself because the circuitry was considered proprietary. This may have lead to duplication of efforts and limited progress in understanding the nature of the problem; as a result the development of solutions has primarily been the result of individual effort, rather than a coordinated approach. One solution which has been publicly pursued has been the utilization of frequency-oriented components to discharge a capacitor through an inductor in such a way as to reverse the current to negate or quench the arc. Unfortunately this solution acts to increase the current in the arc initially and thus can intensify the problem before solving it. Again, this solution is undesirable especially in refined processing environments, and none of the above solutions work adequately well when the process is reactive and the reaction product is a good insulator. For this situation dc-powered reactive sputtering has generally been abandoned in favor of other, more expensive approaches (such as radio-frequency sputtering) because of the lack of a suitable arc-handling solution.
The present invention acts to virtually eliminate the occurrence of arcs in all processes but especially in extremely demanding processing environments such as the sputtering of highly insulating films by reactive sputtering of a conductive target using direct current process power. It does so by periodically clearing uneven buildup of charges and thereby eliminating the original source of the arc. It also acts to avoid processing limitations and effects to a significant extent by interrupting the supply of power or even reversing the voltage regardless of the exact cause of the problem.